1. Field of the Invention
The present invention relates generally to methods and apparatus for delivering drug therapy to the human body. More particularly, the invention relates to implantable apparatus and methods for delivering drug therapy using a recirculating carrier fluid.
2. Description of the Related Art
In conventional implantable drug delivery devices, drug is dissolved in a carrier fluid and the drug/carrier fluid combination is stored in a reservoir and a closely controlled dosage metered and delivered to the body. Drug concentration in the carrier fluid is typically kept very low. One reason for this is that many drugs become unstable at higher concentrations or may precipitate out of the carrier fluid, resulting in blockage within the delivery device. Another reason is that high concentrations of drug may lead to local toxic effects during delivery and would require very low infusion rates beyond the controllable range of many delivery devices. Also, inadvertent leakage of carrier fluid bearing large concentrations of drug would present a danger to the human body.
Since the concentration of drug in the carrier fluid is typically very low, often on the order of a few micrograms to a few milligrams per milliliter of carrier, relatively large volumes of carrier fluid must be stored within the implantable drug delivery device and periodically replenished by refilling from sources external to the human body. For example, in the SYNCHROMED.RTM. infusion pump, manufactured by Medtronic, Inc. of Minneapolis, Minnesota, at least half of the total device volume is occupied by the pump reservoir, which stores carrier and drug. In a morphine administering application, morphine sulfate is dissolved in water at a concentration of less than 50 milligrams per milliliter. Thus, less than 5-percent of the fluid stored in the pump reservoir is actually active drug. In prior art devices, further reduction of the pump size, although desirable, is not achievable without corresponding loss in drug storage capacity.
Some known systems, such as the one disclosed in U.S. Pat. No. 5,643,207, have utilized endogenous body fluids as carrier fluids in order to provide more compact delivery devices. However, such systems present difficulties in controlling the concentration of drug in the physiological fluids and, due to the chemical make-up of physiological fluids, are limited in the types of drug that may be delivered using such systems. There is thus a need for a drug delivery system which eliminates the need for large storage capacity yet which is capable of extended delivery without refilling.